Add the padd storeOp and unify statistics keeping.

Add the padd storeOp and unify statistics keeping.

Filling with zeroes is done within a storeop of its own. This raises
consistency and readability.
Also, the formerly copy-pasted (to afs_dcache and afs_segments respectively)
statistics code is now in a seperate function of its own.

Reviewed-on: http://gerrit.openafs.org/122
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>